Generally, this kind of derailleur provides a tension spring interposed between the movable member and the changeover frame, so that the tension spring always exerts tension on the driving chain when switched to any speed change stage.
When the tension spring, is weak, it cannot exert full tension on the driving chain, so that the slackened chain vibrates greatly at its slack side during the bicycle's running, resulting in the chain falling off a front gear.
On the other hand, a tension spring having a strong force exerts greater tension on the chain so as to cause a deflection in support shafts for front and rear gears carrying the chain. As a result, a large frictional resistance is generated at the relative-rotating portions between rotary members rotatable together with the front and rear gears and fixing members supporting the rotary members. Hence, the problem is created in that the front and rear gears develops a poor characteristic rotating and the relative-rotating portions increasingly wear and generate a large amount of noise. Also, a large tension spring force tension and large driving chain tension allow the chain to greatly large change in tension when switched at a stroke from the highest speed stage to the lowest and vice versa, resulting in poor speed change efficiency.